Mechanism of inhibition of tannic acid and related compounds on the growthof intestinal bacteria

Tannic acid, propyl gallate and methyl gallate, but not gallic acid, were f ound to be inhibitory to the growth of intestinal bacteria Bacteroides frag ilis ATCC 25285, Clostridium clostridiiforme ATCC 25537, C. perfringens ATC C 13124, C. paraputrificum ATCC 25780, Escherichia coil ATCC 25922, Enterob acter cloacae ATCC 13047, Salmonella typhimurium TA98 and S. typhimurium YG 1041 at 100-1000 mu g/ml in culture broth. Neither Bifidobacterium infantis ATCC 15697 nor Lactobacillus acidophilus ATCC 4356 was inhibited by any of the above compounds up to 500 mu g/ml. Tannic acid has a much greater rela tive binding efficiency to iron than propyl gallate, methyl gallate or gall ic acid. The inhibitory effect of tannic acid to the growth of intestinal b acteria may be due to the strong iron binding capacity of tannic acid; wher eas the effect of propyl gallate and methyl gallate probably occurs by a di fferent mechanism. The growth of E. coli was restored by the addition of ir on to the medium after the precipitate caused by tannic acid was removed. N either B. infantis nor L. acidophilus require iron for growth. This probabl y contributes to their resistance to tannic acid. Because tannins are abund ant in the human diet, tannins may affect the growth of some intestinal bac teria and thus may have an impact on human health. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.